The polyphenylene ethers and processes for their preparation are known in the art and described in numerous publications including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, both of which are hereby incorporated by reference. Other patents which show the preparation of polyphenylene ethers include Price et al., U.S. Pat. No. 3,382,212 and Kobayashi et al, U.S. Pat. No. 3,455,880, which are also incorporated by reference. Patents which show the recovery of polyphenylene ethers include Bennett and Cooper, U.S. Pat. No. 3,838,102 and Floryan and Watson, U.S. Pat. No. 3,951,917, both of which are incorporated by reference.
The processes most generally used to produce the polyphenylene ethers involve the self-condensation of a monovalent phenol, such as 2,6-xylenol, in the presence of an oxygen-containing gas and a catalyst comprising a copper-amine complex.
On polymerization of the phenol to form the polyphenylene ether, e.g., 2,6-xylenol to form poly(2,6-dimethyl-1,4-phenylene)ether, and using a copper-amine catalyst, e.g., copper-dibutylamine, or copper-N,N'-di-t-butylethylenediaminedimethylbutylamine, the copper must ultimately be removed from the polymerization mixture before isolation of the product in order to achieve acceptable product polymers, in terms of color, stability, and the like. A very efficient way to accomplish removal of the copper catalyst is to use a chelating agent of the amino acid type, such as nitrilotriacetic acid (NTA) or ethylenediamine tetraacetic acid (EDTA), as aqueous solutions of their sodium salts, to extract the copper-containing organic polymer solution, followed by separation of the aqueous copper-amino acid chelate solution.
It is obviously desirable to provide a means to separate the copper from the complex with amino acid chelate in such a way that both can be reused, because these are generally both expensive materials and since disposal is often difficult. The present invention provides such a means.
It has been discovered that the copper catalyst of a polyphenylene ether polymerization can be effectively recovered from the aqueous copper-amino acid chelate extract solution of a polyphenylene ether polymerization mixture by treating the copper-amino acid extract with glucose or a similar reducing sugar in the alkaline medium, preferably, at a moderately elevated temperature. The copper catalyst is recovered as an oxide of copper, e.g., cuprous oxide, in the form of a precipitate, in nearly quantitative yield. Thereafter, the amino acid chelating agent can be recovered from the effluent, after removal of the copper precipitate, by acidifying, e.g., to a pH of less than 2, which causes the precipitation of the amino acid and facilitates its recovery in very high yields.
Upon isolation and recovery of the copper and amino acid, these can readily be recycled. For example, cuprous oxide can be redissolved with aqueous hydrobromic acid or mixtures of hydrobromic acid and bromine to form an active copper catalyst. The amino acid is readily redissolved with aqueous alkali to form a soluble salt, which is then used for subsequent extraction of a polyphenylene ether reaction mixture to remove copper.
Several reports are known concerning the reduction of copper (II) from copper (II)-EDTA and copper (II)-NTA chelates in connection with oxidations of various reducing sugars. See Wagreich and Harrow, Analytical Chemistry, 25, No. 12, p. 1925 (1953); Heunart Chemical Abs., 62, 97976; and Yashida, Murakami and Hayakawa, Chemical Abs., 63, 15084 E. In these reports, however, large excesses of the copper-EDTA and copper-NTA chelates were employed to fully oxidize the sugars as a semiquantitative determination of the amount of sugar present. There is no suggestion that the reaction could form the basis of the instant recovery process. It is surprising to find that a rapid and quantitative separation of copper can be achieved using less than 1 mole of sugar per gram-atom of copper.